Journal of Optics J. Opt. 18 (2016) 033001 (24pp)
doi:10.1088/2040-8978/18/3/033001
Topical Review
Plasmonic nanostructures for organic photovoltaic devices Sungmo Ahn1, Devin Rourke1 and Wounjhang Park1,2 1
Department of Electrical, Computer, and Energy Engineering, University of Colorado, Boulder, CO 80309-0425, USA 2 Materials Science and Engineering Program, University of Colorado, Boulder, CO 80309-0425, USA E-mail:
[email protected] Received 21 August 2015, revised 9 November 2015 Accepted for publication 16 November 2015 Published 9 February 2016 Abstract
Due to the increasing demand on high-efficiency organic photovoltaic (OPV) devices, light management technique has become an active research subject. Especially, plasmonic approach was proven to be suitable for application in OPV and has shown lots of successful results. In this review, we summarize recent studies on plasmonic nanostructures for OPV with their underlying enhancement mechanisms. Optical absorption enhancement by the resonant scattering and the strong plasmonic near field will be discussed for various implementation geometries including metal nanoparticles, patterned electrodes, and plasmonic metamaterials. In addition, we will also look into the electrical effects originating from plasmonic nanostructures, which inevitably affect the device’s efficiency. Future research directions will be also discussed. Keywords: organic photovoltaics, plasmonics, nanostructure, solar cell, organic electronics (Some figures may appear in colour only in the online journal) (PCE) from these organic materials could not exceed 0.1% until the bi-layer structure made of donor and acceptor molecules was demonstrated by Tang in 1986 [17], leading to a dramatic increase of PCE to 1%. The ensuing research on the reason for this improvement revealed that the charge separation from an exciton in an organic semiconductor is far more efficient at the interface of the heterojunction [17–19]. Later, bulk heterojunction of interpenetrating donor–acceptor network was introduced to overcome the short exciton diffusion length which sets a limit in the active layer thickness and consequently in the optical absorption [20]. Bi-layer and bulk heterojunction solar cells are schematically shown in figure 1. In the past decade, polymer:fullerene-based solar cell was extensively studied and showed a great efficiency improvement. The famous Poly(3-hexylthiophene) (P3HT):PhenylC61-butyric acid methyl ester (PCBM) device typically shows 4∼5% PCE [22–24]. In addition, poly(N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) with deep HOMO level was
1. Introduction Organic photovoltaic (OPV) devices have become one of the major technologies in the field of solar energy harvesting. As the silicon or other inorganic semiconductor-based solar cells are under increasing pressure for lower cost [1, 2], OPV offers a promising alternative. Moreover, OPV is compatible with flexible plastic substrates which may open an array of new applications such as foldable and portable devices that can be directly integrated on textile [3], curved surfaces [4–6] or moving parts of robots and organs [7–9]. Semi-transparency is also attractive for a variety of applications including powergenerating and color-decorative windows in buildings and automobiles [10–13]. The history of photovoltaic technology based on organic semiconductors stretches back to the early 20th century when the photoconductivity was first observed from an organic molecule, anthracene [14–16]. Since then, small molecules and polymers have been investigated to realize photovoltaic devices. Unfortunately, the power conversion efficiency 2040-8978/16/033001+24$33.00
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© 2016 IOP Publishing Ltd Printed in the UK
J. Opt. 18 (2016) 033001
Topical Review
Figure 1. Schematics of (a) bilayer and (b) bulk heterojunction organic photovoltaic (OPV) device. (c) Device operating principle of OPV
device from light absorption to charge collection. Reprinted from [21]. Copyright 2014 MDPI.
developed to show PCEs of 6∼7% [25–27]. More recently, narrower bandgap polymers such as polythieno [3,4-b]-thiophene/benzodithiophene (PTB7) and PTB7-Th were used to improve the PCE by absorbing lower-energy photons. Almost 10% PCE were obtained from a single stacked polymer:fullerene solar cell by using PTB7 as donor material [28]. Small molecule-based solar cell has been also improved to achieve a PCE over 9% from single junction recently [29]. Though lots of improvements in material and device structure have been accomplished so far [22, 25, 27, 30–33], the PCE of OPV devices still remains low compared with their inorganic counterparts. According to the efficiency chart reported by National Renewable Energy Laboratory [34], the highest efficiency of OPV marked 11.5% while other inorganic technologies show efficiencies higher than 20%. As mentioned earlier, the active layer thickness in an OPV device is limited up to a few hundred nanometers because of the short exciton diffusion length of organic materials, which inevitably limits optical absorption [35, 36]. This trade-off between the optical absorption and the charge carrier collection makes it difficult to achieve higher PCE in OPV devices. For this reason, light management has emerged as an important area of research. Since the optical absorption
depends on the optical path length and the electromagnetic field strength, it is possible to increase total absorption without increasing the active layer thickness by incorporating photonic structures. For example, a microcavity formed in the active layer resulted in higher interaction between the optical field and the absorbing material, yielding enhanced PCE [37]. To increase the effective absorption length while maintaining small active layer thickness, various nano-photonic approaches have been investigated including photonic crystals [38– 41], quasi-periodic or disordered nano-patterns [42–44], nanoparticles (NPs) [45], and plasmonic nanostructures [46]. In this review, we will focus on the plasmonic nanostructures used in organic solar cells. During the past decade, surface plasmon has become an important technique in the field of solar cells [47, 48]. Surface plasmon is a collective oscillation of surface charges, which can interact strongly with external electromagnetic waves [49]. Due to the nearfield nature of surface plasmons [50], they are intrinsically suitable for thin film devices. Photonic crystals, which have shown great successes in inorganic solar cells, have been also applied in OPV devices [41, 51, 52]. However, this approach will be limited by the relatively low refractive indices of organic materials since the strong photonic crystal effect can 2
J. Opt. 18 (2016) 033001
Topical Review
be seen with high index contrast and also with a large enough thickness to support the guided modes [53, 54]. Therefore, plasmonic nanostructures are considered better suited for application in OPV devices. Metal NP and patterned metal electrode are two most popular plasmonic nanostructures used in OPV devices. Metal NPs can impact OPV performance by enhanced scattering and local field enhancement resulting from the excitation of localized surface plasmon resonances (LSPR). Patterned electrodes may improve OPV through local field enhancement and increased optical path length by coupling into the surface plasmon polaritons (SPPs). These effects will be first discussed and the relevant research works will be summarized. Also, the recent studies on plasmonic metamaterials will be highlighted with their future applications in OPV technology. In the second part of this review, the electrical effects of plasmonic nanostructures incorporated in OPV devices will be discussed, including morphological change of bulk heterojunction solar cell after the metal NP incorporation and the effects on the charge collection efficiencies of non-uniform plasmonic fields across the active layer. It is noted that the various plasmonic effects described in this paper are not mutually exclusive and a structure may exhibit more than one plasmonic effect simultaneously. In fact, designing a plasmonic structure making use of multiple plasmonic effects to achieve maximum enhancement is one of the main future research directions. It is also noted that there were recent reviews [55–57] on the plasmonic enhancement of OPV and this paper attempts to provide more up-to-date advances and insights in this field.
2.1. Metal nanoparticles
Metal NPs are one of the most intensively studied plasmonic structures due to the LSPRs which may be controlled by their size and shape [49, 60]. As opposed to the patterned metal electrode we will discuss in the next section, the relative easiness of fabrication and the solution processibility lead many researchers to use metal NPs in various applications such as light-emitting devices [61–63], solar energy harvesting [48, 64, 65], bio-imaging and medical therapies [66], etc. OPV is also an important application area where the metal nanoparticles can improve the absorption efficiency through their plasmonic resonances. When the metal NPs are incorporated in solar cell, both the enhanced scattering and the strong near field of surface plasmons can contribute to the overall efficiency enhancement. Light scattering by small particles has been a subject of interest in optics for a long time. When the NP size is much smaller than the wavelength of light, the scattering (Csc) and the absorption (Cabs) cross sections from a spherical NP can be expressed as 2
Csc = pa2Qsc, Cabs = pa2Qabs ,
Qsc =
8 e -1 (ka)4 d , 3 ed + 2
⎡ e - 1⎤ Qabs = 4 (ka) Im ⎢ d ⎥, ⎣ ed + 2 ⎦
where k is the wave number, a is the particle radius, and εd=εp/εm is the relative permittivity given by the ratio between the permittivity of the particle (εp) and the surrounding medium (εm) [67]. One can notice from the above equations that these cross sections are enhanced at the Fröhlich condition of Re[εd]=−2, which represents the excitation of LSPR. The general solution for the light scattering by a spherical NP is given by the Mie theory, which contains all of the higher order terms that become nonnegligible for large particle sizes [68]. On the other hand, when the absorbing active materials are in close proximity to the metal surface, the exciton generation rate can also be enhanced as proportional to the square of the local field enhancement factor as follows
2. Optical effects In this section, the optical effects from surface plasmon resonance in OPV devices will be discussed. When plasmonic structures are incorporated into photovoltaic devices, absorption enhancement can be achieved by two different mechanisms. One is the scattering effect, known to be stronger in metal NPs which support LSPRs. Enhanced scattering at the front surface of a photovoltaic device reduces the Fresnel reflection while the scattering inside the absorbing layer makes the effective optical path length much longer than the physical thickness of the absorber layer [47, 58, 59]. The other mechanism is the local field enhancement effect of surface plasmons. Surface plasmons are known to be highly effective in concentrating electric field at the interface between the metal and the dielectric material. When this concentrated field is present within the active material, optical absorption will be increased proportionally to the square of the field enhancement factor. The contribution from each mechanism on the absorption enhancement depends on the type of plasmonic structures and their different geometries as schematically shown in figure 2. Therefore, in the following subsections, we will discuss the plasmonic optical effects in OPV devices according to their geometries: the metal NPs and the patterned metal electrodes.
GµP=
1 e w E 2 , 2
where G is the exciton generation rate, P is the electromagnetic energy dissipation rate, e is the imaginary part of the complex permittivity of the active material, w is the angular frequency of the electromagnetic wave, and E is the local electric field. For the past decade, there has been a substantial amount of theoretical and experimental studies about the plasmonic enhancement of OPV device performance using metal NPs. Polymer bulk heterojunction solar cells as well as small molecule based devices have shown a successful efficiency enhancement from plasmonic metal NP inclusion [69–72]. The experimental studies so far have shown the PCE enhancement of a few tens of percent using various types of metal NPs. The origin of the enhancement is believed to be a 3
J. Opt. 18 (2016) 033001
Topical Review
Figure 2. Plasmonic light trapping geometries in thin film solar cell. Light trapping (a) by scattering from metal nanoparticles (NPs) which
increases the effective light path length, (b) by the excitation of localized surface plasmons creating strong near field around the metal NPs, (c) by the excitation of surface plasmon polaritons propagating laterally along the metal-dielectric interface. Reprinted with permission from [47]. Copyright 2010 Nature Publishing Group.
combination of the enhanced scattering and the local field enhancement as we will discuss in detail below. The contributions from these two different mechanisms depend strongly on the exact geometry of metal NP-embedded in the OPV device. As shown in the equations for the scattering and the absorption cross sections, the scattering increases much more rapidly than the absorption as the particle size is increased. Therefore, scattering enhancement is dominant for large size NPs while the local field enhancement effect is prevalent for NPs with small diameters. A comprehensive list of theoretical and experimental studies is given in table 1. Metal NP’s position is an important factor that affects the enhancement mechanisms of plasmonic OPV devices. In many cases, metal NPs are placed inside the anodic buffer layer such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as shown in figure 3(a). Various fabrication methods like electrodeposition [74], thin film evaporation [73] and chemical synthesis [100] were used to incorporate metal NPs into OPV devices. The PCE enhancement ranging from 5%∼69% has been reported experimentally. Morfa et al reported 69% efficiency improvement of P3HT:PCBM device from vapor-phase deposited Ag nanoparticles on ITO, but the overall efficiency remained low at 2.23% [73]. Generally, when the original efficiency is low, it becomes easier to achieve large enhancement. With moderate original efficiencies, it seems that ∼20% efficiency enhancement is the best experimentally observed enhancement so far [74–77, 90, 91, 96]. To identify the origin of this PCE enhancement, the external quantum efficiency (EQE) spectrum is often measured and compared with the metal NPs’ extinction spectrum as shown in figure 4. The EQE also called incident photon to current conversion efficiency (IPCE), is defined as the ratio between the number of electrons collected and the number of photons incident on the device. It can be expressed as the product of the absorption efficiency and the charge collection efficiency. Assuming that the charge collection is not affected by the plasmonic structure or at least it does not have wavelength dependence, the spectral overlap between the metal NP’s extinction and the EQE enhancement is an indication that the optical absorption enhancement by the metal NP LSPR is the main reason for the PCE enhancement.
Since the metal NPs embedded in buffer layer are not in direct contact with the active layer, it is believed that the forward scattering of the incident sunlight is mainly responsible for the absorption enhancement. Lee et al pointed out that the spatial extent of the absorption enhancement around metal NP is so small that it exponentially decays away and reaches below 10% at distances of only 10∼20 nm from the metal surface [214]. It is also theoretically shown that the plasmonically enhanced local field of metal NPs embedded in buffer layer does not extend into the active layer [80]. These studies all support the conclusion that the forward scattering is the main mechanism of the efficiency enhancement from metal NPs embedded in buffer layer. However, in many experimental studies, the metal NP’s size is comparable to the thickness of PEDOT:PSS layer or even larger than that. In this case, even the highly concentrated plasmonic field around the metal NP can contribute to the absorption enhancement of the active layer. Several studies have shown that the metal NPs positioned at the interface between the active and the buffer layer lead to higher enhancement [120–122]. From Qu et al’s theoretical study, Ag nanospheres embedded in between P3HT:PCBM and PEDOT:PSS layer could result in absorption enhancement as high as 108% [121]. Incorporating metal NPs in the buffer layer seems to be a good way to get enhancement from plasmonic effect without deteriorating the interfacial morphology of blended active layer or quenching of photogenerated excitons [102], which could adversely affect the device performance. But it has also been reported that the PCE can be enhanced from the metal NPs directly embedded in the active layer as shown in figure 3(b). Since the metal NP is in direct contact with the absorbing material in this case, one can expect higher near field effect than the metal NPs embedded in buffer layer. Up to about 40% efficiency enhancement was achieved so far [107, 109]. Wang et al reported ∼32% PCE enhancement in polymer solar cell with Au nanoparticles incorporated in the active layer [108]. Numerically solving Maxwell’s equations, the near field profile at LSPR was revealed to show very strong field strength laterally distributed along the active layer as in figure 5(b). An absorption enhancement of over 100% at LSPR wavelength was experimentally achieved and matched well with the theoretical expectation as shown in figure 5(a). 4
J. Opt. 18 (2016) 033001
Topical Review
Table 1. List of theoretical and experimental studies about plasmonic OPV devices utilizing metal nanoparticles.
Donor:Acceptor
Type of metal nanoparticle (NP)
Position
P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM MEH-PPV:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM
Ag nanoisland Ag NP Au nanosphere Au nanodots Au nanosphere Au-Cu alloy NP Au nanosphere Au nanosphere Au NP Au NP-graphene oxide Au nanosphere, nanorod Au nanowire Ag NP Ag nanosphere
Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer Buffer
P3HT:PCBM P3HT:PCBM P3HT:ICBA PCDTBT:PCBM PTB7:PCBM PTB7:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM P3HT:ICBA PCDTBT:PCBM PCDTBT:PCBM PTB7:PCBM PCDTBT:PCBM PCDTBT:PCBM PTB7:PCBM PTB7:PCBM P3HT:PCBM P3HT:PCBM PTB7:PCBM PTB7:PCBM ZnPc:C60
Ag nanoprism Au NP Ag NP Ag nanosphere Ag nanosphere Au, Ag nanosphere Pt NP Ag NP, nanodot Au NP-graphene oxide Au NP-graphene oxide Ag NP Au@Ag core–shell nanocube Au@Ag core–shell nanocube Ag nanosphere Au nanosphere Au nanosphere Au NP Au NP Cu NP Cu NP Au nanocube Ag NP
F4-ZnPc:C60
Ag NP
P3OT:C60 P3HT:PCBM P3HT:PCBM PCDTBT:PCBM Si-PCPDTBT:PCBM PCDTBT:PCBM P3HT:PCBM P3HT:PCBM P3HT:PCBM PFSDCN:PCBM P3HT:PCBM PCDTBT:PCBM P3HT:PCBM PCPDTBT:PCBM P3HT:PCBM
Ag NP Au nanosphere Octahedral Au NP Octahedral Au NP Octahedral Au NP Ag NP Ag nanowire AuAg alloy nanosphere Au NP Au nanosphere Ag nanoplate Ag nanoplate Au@SiO2 core–shell nanorod Au@SiO2 core–shell nanorod Combination of Ag nanosphere and nanoprism Al NP Au@SiO2 core–shell nanorod Au@SiO2 core–shell nanorod
Buffer (PEDOT:PSS) Buffer (MoO3) Buffer (MoO3) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer Buffer Buffer Buffer (MoO3) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (MoS2) Buffer (ZnO) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (PEDOT:PSS) Buffer (BF-DPB:F6TCNNQ) Buffer (BF-DPB:F6TCNNQ) Active Active Active Active Active Active Active Active Active Active Active Active Active Active Active
4.36 6.45 4.54 7.1 3.91 4.73 3.71 2.17 4.4 6.6 3.58 4.4 4.3
Active Active Active
P3HT:PCBM P3HT:PCBM PBDTT-DPP:PCBM
5
(PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (Cs2CO3) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS) (PEDOT:PSS)
PCE (%)
Enhancement factor
2.2 (1.3) 1.69 3.69 (3.05) 1.21 4.19 (3.48) 1.20 3.65 (3.04) 1.20 4.24 (3.57) 1.19 3.35 (2.90) 1.16 2.36 (1.99) 1.19 3.51 (3.1) 1.13 3.54 (3.12) 1.13 3.55 (3.23) 1.10 4.28 (3.46) 1.24 2.72 (2.44) 1.11 2.82 (2.41) 1.17 34% absorption enhancement, theoretical 5.21 (4.58) 1.14 4.20 (3.68) 1.14 7.21 (6.26) 1.15 7.6 (6.4) 1.19 8.6 (7.9) 1.09 8.67 (7.25) 1.20 2.57 (2.29) 1.12 4.80 (4.02) 1.19 3.98 (3.26) 1.22 5.05 (4.02) 1.26 5.87 (5.07) 1.16 6.3 (5.3) 1.19 9.2 (8.0) 1.15 7.35 (6.50) 1.13 6.01 (5.29) 1.14 8.31 (7.95) 1.05 7.25 (6.18) 1.17 3.6 (2.3) 1.57 3.96 (3.58) 1.11 7.43 (6.79) 1.09 8.2 (7.5) 1.09 2.65 (1.93) 1.37 3.4 (2.7)
1.26
1.9 (1.1) (3.54) (5.77) (3.92) (6.3) (3.31) (3.61) (2.64) (1.64) (3.2) (5.9) (3.17) (3.5) (3.6)
1.73
